1. Field of the Invention
The present invention relates to an optical system and an optical apparatus including the optical system, and is suitable for use in an optical apparatus such as a silver-halide film camera, a digital still camera, a video camera, a telescope, binoculars, a projector, or a copying machine.
2. Description of the Related Art
An optical system used for an optical apparatus such as a digital camera or a video camera is required to have a short total lens length (length between first lens surface located on object side and image plane) and high optical performance.
It is desired that a change in optical performance be small even under environmental changes such as changes in temperature, atmospheric pressure, and humidity.
In general, along with a further reduction in size of the optical system, various aberrations, for example, chromatic aberrations such as axial chromatic aberration and lateral chromatic aberration, are increasingly generated to degrade the optical performance.
For many optical systems, in order to reduce the chromatic aberrations while shortening the total lens length, for example, it is desirable to use a lens made of a material having extraordinary partial dispersion, such as a fluorite, and increase a refractive power of the lens. However, when a refractive power of a lens made of low-dispersion glass having a large Abbe number, such as the fluorite, is to be adjusted by a large amount to correct the chromatic aberrations, various aberrations other than the chromatic aberrations, for example, spherical aberration, coma, and astigmatism are increasingly generated.
An optical system using an optical element made of an optical material having strong extraordinary partial dispersion, for example, an organic material, has been proposed as the optical system having reduced chromatic aberrations (U.S. Pat. No. 7,292,398).
According to U.S. Pat. No. 7,292,398, the optical element made of the organic material having strong extraordinary partial dispersion is given a refractive power to achieve a chromatic aberration balance with another optical element made of glass, thereby reducing the chromatic aberrations of the entire optical system.
However, a refractive index change dn/dt of the organic material such as plastic takes a larger value than that of glass with respect to a change in temperature, and is, for example, approximately 10 times to 200 times larger than that of glass.
Therefore, when the optical element which is made of the organic material provided with a strong refractive power is used, it is important to reduce focal and aberration variations of the optical system caused by the change in temperature.
An optical system in which a plurality of plastic lenses are combined to compensate for the focal variation caused by the change in environmental temperature has been proposed as the optical system using the optical element made of the organic material (Japanese Patent Application Laid-Open No. S63-285511). However, it is difficult to correct the chromatic aberrations of the optical system with the method disclosed in Japanese Patent Application Laid-Open No. S63-285511.
Therefore, when the chromatic aberrations of the optical system are corrected using the optical element made of the organic material, it is also important to reduce the variations in chromatic aberrations caused by the environmental changes, particularly, the change in temperature in order to maintain high optical performance.
For example, when the optical element made of the organic material having the extraordinary partial dispersion characteristic is used for the optical system, it is important to suitably set a position on the optical path and a refractive power.
When such conditions are unsuitable, it is difficult to maintain high optical performance under the environmental changes while the entire optical system is reduced in size.